“I could not imagine how it could happen here”.
Two very young film makers, Kevin Brownlow and Andrew Mollo, did sort of imagine it in their counter-factual film “It Happened Here” (1964). It imagined a Britain which had been invaded and occupied, with some people choosing to be collaborators.
It was highly controversial at the time, so much so that United Artists cut 10 minutes out of the film (which Brownlow put back in when he regained the rights many years later).
The film took 8 years to make, with filming happening whenever Brownlow was able to cobble together enough money. He later quipped that it took longer to film than the war took to fight.